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- // Boost.TypeErasure library
- //
- // Copyright 2011 Steven Watanabe
- //
- // Distributed under the Boost Software License Version 1.0. (See
- // accompanying file LICENSE_1_0.txt or copy at
- // http://www.boost.org/LICENSE_1_0.txt)
- //
- // $Id$
- #include <boost/type_erasure/any.hpp>
- #include <boost/type_erasure/any_cast.hpp>
- #include <boost/type_erasure/builtin.hpp>
- #include <boost/type_erasure/operators.hpp>
- #include <boost/type_erasure/tuple.hpp>
- #include <boost/mpl/vector.hpp>
- #include <iostream>
- namespace mpl = boost::mpl;
- using namespace boost::type_erasure;
- void multi1() {
- //[multi1
- /*`
- Operations can have more than one __any argument.
- Let's use binary addition as an example.
- */
- typedef any<
- mpl::vector<
- copy_constructible<>,
- typeid_<>,
- addable<>,
- ostreamable<>
- >
- > any_type;
- any_type x(10);
- any_type y(7);
- any_type z(x + y);
- std::cout << z << std::endl; // prints 17
- /*`
- This is /not/ a multimethod. The underlying types of the
- arguments of `+` must be the same or the behavior is undefined.
- This example is correct because the arguments both hold
- `int`'s.
- [note Adding __relaxed leads an exception rather than undefined
- behavior if the argument types are wrong.]
- */
- //]
- }
- void multi2() {
- //[multi2
- /*`
- __addable`<>` requires the types of the arguments to be exactly
- the same. This doesn't cover all uses of addition though. For
- example, pointer arithmetic takes a pointer and an integer and
- returns a pointer. We can capture this kind of relationship among
- several types by identifying each type involved with a placeholder.
- We'll let the placeholder `_a` represent the pointer and the
- placeholder `_b` represent the integer.
- */
- int array[5];
- typedef mpl::vector<
- copy_constructible<_a>,
- copy_constructible<_b>,
- typeid_<_a>,
- addable<_a, _b, _a>
- > requirements;
- /*`
- Our new concept, `addable<_a, _b, _a>` captures the
- rules of pointer addition: `_a + _b -> _a`.
- Also, we can no longer capture the variables
- independently.
- ``
- any<requirements, _a> ptr(&array[0]); // illegal
- ``
- This doesn't work because the library needs
- to know the type that _b binds to when it
- captures the concept bindings. We need to
- specify the bindings of both placeholders
- when we construct the __any.
- */
- typedef mpl::map<mpl::pair<_a, int*>, mpl::pair<_b, int> > types;
- any<requirements, _a> ptr(&array[0], make_binding<types>());
- any<requirements, _b> idx(2, make_binding<types>());
- any<requirements, _a> x(ptr + idx);
- // x now holds array + 2
- /*`
- Now that the arguments of `+` aren't the same type,
- we require that both arguments agree that `_a` maps
- to `int*` and that `_b` maps to `int`.
- We can also use __tuple to avoid having to
- write out the map out explicitly. __tuple is
- just a convenience class that combines the
- placeholder bindings it gets from all its arguments.
- */
- tuple<requirements, _a, _b> t(&array[0], 2);
- any<requirements, _a> y(get<0>(t) + get<1>(t));
- //]
- }
- //[multi
- //` (For the source of the examples in this section see
- //` [@boost:/libs/type_erasure/example/multi.cpp multi.cpp])
- //` [multi1]
- //` [multi2]
- //]
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